No abstract

It is shown that the incommensurate (IC) phase near the α ↔ β transition point in quartz (SiO 2 ) is an improper ferroelastic one and should be split into domains. The birefringence induced by such a ferroelastic phase is estimated, and it is shown that it can be as strong as is observed in experiments only if one accepts a new model for the IC transition in quartz.

No abstract

We show that a ferroelectric phase transition takes place in the incommensurate phase of the K 2 SeO 4 crystal. The ferroelectric character of the incommensurate phase explains the second-harmonic generation observed in the corresponding temperature range.